1. Field of the Invention
This invention relates to an ignition device for igniting a fuel-air mixture supplied to an engine. More particularly, it relates to an engine ignition device which is adapted so that an ignition timing may be advanced or delayed in response to a change in an engine speed.
2. Description of the Related Art
A small-sized simple general-purpose engine has been heretofore used as a drive source in a small-sized working machine or the like including a mower, for example. In this type of general-purpose engine, a self-trigger type ignition device having a relatively simple constitution is used in order to ignite a fuel-air mixture supplied to a combustion chamber.
This type of ignition device comprises a spark plug disposed in the combustion chamber of the engine; a flywheel disposed in a crankshaft of the engine; a magnet disposed in the flywheel; and a cored ignition coil adjacent to a revolution locus of the magnet and connected to the spark plug through a high-tension cord.
The magnet is revolved in synchronization with a rotation of the crankshaft. The magnet passes besides the core, whereby a high voltage is generated in the ignition coil. This high voltage is supplied to the spark plug, whereby sparks are generated in the spark plug, so that the fuel-air mixture supplied to the combustion chamber is ignited. In such an ignition device, the timing of the generation of the sparks in the spark plug, i.e., the timing of the ignition of the fuel-air mixture is a constant timing to a rotation cycle of the crankshaft, regardless of a difference in a rotational speed of the crankshaft. Due to this, when the rotational speed of the crankshaft, namely, an engine speed is high, the ignition timing may be too late to obtain a sufficient engine output. When the engine speed is low, the ignition timing may be so early that an idle operation is unstable. Thus, this type of ignition device also requires a function of advancing or delaying the ignition timing in response to the difference in the engine speed.
Japanese Patent Publication No. 61-10672 discloses an example of a trigger type ignition device capable of advancing the ignition timing (hereinafter referred to as "the first prior art"). This ignition device has a control circuit for controlling the ignition timing. The control circuit includes an advance capacitor. The engine speed is increased, whereby a charging potential is increased in the advance capacitor. The timing when a main transistor for actuating the ignition coil is turned on is thus advanced. The timing of the ignition of the spark plug by the ignition coil is consequently advanced.
On the other hand, Japanese Patent Application Laid-open No. 1-262367 discloses another example of the trigger type ignition device capable of advancing the ignition timing (hereinafter referred to as "the second prior art"). In the engine incorporating a flywheel/magnet type generator, this ignition device allows a stator of the generator to be rotatably supported by an electromagnetic force generated between the stator and the magnet on the side of the flywheel rotated in synchronization with the engine. The ignition device has a spring member for suppressing the rotation of the stator in a direction of rotation of the flywheel. The ignition device has a drive mechanism for moving a self-trigger type ignition unit (including a pickup core, etc.) sharing the flywheel. This drive mechanism moves the ignition unit in the opposite direction to the direction of rotation along an outer periphery of the flywheel in response to the rotation of the stator. The ignition unit is moved in response to the difference in the engine speed, whereby the ignition timing of the engine is advanced or delayed.
However, the ignition device according to the first prior art tends to have a relatively narrow advance range of the ignition timing which can be changed by the advance capacitor. Moreover, for a mass-production of the advance capacitor, the set values of time constant associated with the charge of the advance capacitor may be varied and thus the advance values of the ignition timing may be varied.
On the other hand, the ignition device according to the second prior art tends to have a relatively large ignition unit and thus to have a complicated large-sized drive mechanism and support mechanism for moving the ignition unit. Thus, the ignition device is structurally unstable and also has a problem in operation reliability.